91 Bravada 4.3 liter has extreme high idle speed

Yesterday...warm outside temp. but cold start for motor. Had to feather the accelerator to get it started and to keep it running. Engine choked off twice prior to leaving my driveway. Drove for 30 minutes mostly stop and go. Engine reached operating temp. At a stoplight engine began to race and I had to use extra braking force to keep from rolling forward. Did this once more at next stoplight. Reached my destination and vehicle idled correctly in parking lot. Cycled start several times over a few minute period and condition did not repeat. Vehicle sat for 20 minutes....restarted to leave and fast idle reoccurred and was consistent at every stoplight on the return trip home. Have had this problem for several years and the shop has replaced a sensor, but since this is not a daily driver vehicle, so I basically have learned to live with it and have not noticed any difference since they replaced the sensor. I would have to review my service receipt but I think it was a temp sensor. It is a nuisance problem in that the high engine revs scares others from driving this vehicle and I am sure my gas milage and brake life are taking a hit. Any ideas as to what I can have my mechanic try? I think that I have the 4.3 non-Vortec motor.

It'll likely be anything that can leak air into the engine...broken vacuum hoses, sticking Idle Air Control valve (IAC) or even a sticking throttle.